# Karim Adiprasito: Flag simplicial complexes and the non-revisiting path conjecture

This post is authored by Karim Adiprasito

The past months have seen some exciting progress on diameter bounds for polytopes and polytopal complexes, both in the negative and in the positive direction.  Jesus de Loera and Steve Klee described simplicial polytopes which are not  weakly vertex decomposable and the existence of non weakly k-vertex decomposable polytopes for k up to about $\sqrt{d}$ was proved  by Hähnle, Klee, and Pilaud in the paper  Obstructions to weak decomposability for simplicial polytopes. In this post I want to outline a generalization of a beautiful result of Billera and Provan in support of the Hirsch conjecture.

I will consider the simplicial version of the Hirsch conjecture, dual to the classic formulation of Hirsch conjecture. Furthermore, I will consider the Hirsch conjecture, and the non-revisiting path conjecture, for general simplicial complexes, as opposed to the classical formulation for polytopes.

Theorem [Billera & Provan `79] The barycentric subdivision of a shellable simplicial complex satisfies the Hirsch Conjecture.

The barycentric subdivision of a shellable complex is vertex decomposable. The Hirsch diameter bound for vertex decomposable complexes, in turn, can be proven easily by induction.

This is particularly interesting since polytopes, the objects for which the Hirsch conjecture was originally formulated, are shellable. So while in general polytopes do not satisfy the Hirsch conjecture, their barycentric subdivisions always do! That was a great news!

Shellability is a strong combinatorial property that enables us to decompose a complex nicely, so it does not come as a surprise that it can be used to give some diameter bounds on complexes. Suprisingly, however, shellability is not needed at all!  And neither is the barycentric subdivision!

A simplicial complex Σ is called flag if it is the clique complex of its 1-skeleton. It is called normal if it is pure and for every face F of Σ of codimension two or more, Lk(F) is connected.

Theorem (Adiprasito and Benedetti):  Any flag and normal simplicial complex Σ satisfies the non-revisiting path conjecture and, in particular, it satisfies the Hirsch conjecture.

This generalizes the Billera–Provan result in three ways:

— The barycentric subdivision of a simplicial complex is flag, but not all flag complexes are obtained by barycentric subdivisions.

— Shellability imposes strong topological and combinatorial restrictions on a complex; A shellable complex is always homotopy equivalent to a wedge of spheres of the same dimension, and even if a pure complex is topologically nice (if, for example, it is a PL ball) it may not be shellable, as classic examples of Goodrick, Lickorish and Rudin show. Being normal still poses a restriction, but include a far wider class of complexes. For example, every triangulation of a (connected) manifold is normal, and so are all homology manifolds.

— Instead of proving the Hirsch conjecture, we can actually obtain the stronger conclusion that the complex satisfies the non-revisiting path conjecture, which for a given complex is stronger than the Hirsch conjecture.

A geometric proof of our theorem appeared in a recent paper “Metric geometry and collapsibility”  with Bruno Benedetti. . I will give here a short combinatorial proof.

# Tokyo, Kyoto, and Nagoya

Near Nagoya: Firework festival; Kyoto: with Gunter Ziegler; with Takayuki Hibi, Hibi, Marge Bayer, Curtis Green and Richard Stanly; Tokyo: Peter Frankl; crowded crossing. Added later: Mazi and I at the same restaurant taken by Stanley.

I just returned from a trip to Japan to the FPSAC 2012 at Nagoya and a workshop on convex polytopes in Kyoto. As in my first visit to Osaka in 1999 I found Japan stunning, and this time I was able to share the experience with my wife.

Kyoto: The week before FPSAC there was a workshop at RIMS devoted to convex polytopes. Some highlights: A classical result by Venkov and McMullen characterizes polytopes whose translates tile the Euclidean d-space.  Sinai Robins talked about his work with Nick Gravin and Dima Shiryaev about k-tilings (every point is covered k times).  Not a full characterization yet but already some impressive results. Gunter Ziegler talked about his work with Karim Adiprasito disproving an old conjecture by Shephard which asserts that there are only finitely many projectively unique d-polytopes for every dimension d. This is false above dimension 81.  Eran Nevo talked about his solution with Satoshi Murai of the generalized lower bound conjecture and some subsequent works on triangulated manifolds. There were several talks relating Ehrhard polynomials of polytopes with enumerative combinatorics,  and several talks on algebraic geometry, toric varieties, Fano varieties, and mirror symmetry. Here are the slides of my lecture entitled open problems on convex polytopes I’d love to see solved (but some further explanations for some parts are needed). I hope to return to some of the topics of the workshop in a later post.

Nagoya: FPSAC (Formal power series and algebraic combinatorics) is a central annual  combinatorial meeting with special emphasis on enumerative and algebraic combinatorics. This year it was the 24th such event although I could have swear that I took part in an FPSAC in Montreal already in 1985 but apparently this was a conference with a similar flavor and different name. Much is going on in enumerative and algebraic combinatorics. Cluster algebras, miraculously discovered a decade ago by Fomin and Zelevinsky are going strong in combinatorics as well as in other areas. Alternating sign matrices continue to offer amazing problems and answers. There were quite a few lectures on representation theory, symmetric functions, random matrices, and also on relations of enumerative combinatorics with hyperplane arrangements with polytopes and with physics. There were lectures involving massive computations and new computerized method and packages for symbolic computations  were described. Here are the slides of my lecture entitled Discrete isoperimetry: problems, results, applications and methods. The program page now contains slides for most lectures.

### What are alternating sign matrices?

I suppose that you all know what a permutation matrix is (an n by n matrix with 0,1 entries and one non zero entry in each row and each column) and alternating sign matrices are amazing generalization of permutation matrices discovered by William Mills, David Robbins, and Howard Rumsey. (See also this article by Bressoud and Propp) They are integral n by n matrices with 0,1 and -1 entries. In every row and every columns  the non zero aliments (and there must be at least one such element)  should alternate between 1 and -1 and start and end with a ‘1’. Alternating sign matrices are in one to one correspondence with monotone triangle. Thos are triangles of integers starting with a row: 1,2,…,n. With the following rules: (a) all rows are strictly increasing; (b) every element in row i is weakly between the two elements above it.

The amazing thing is that the number of alternating sign matrices of order n is precisely

## 1!4!7!…(3n-2)!/n!(n+1)!…(2n-1)!

This was a conjecture by Mills, Robbins and Rumsey and it took over a decade until it was proved by Doron Zeilberger. Later Greg Kuperberg found a short proof and by now several proofs are known. If you have some information or ideas on alternating sign matrices that you would like to share or some questions about them, or about anything else, please contribute a comment.

# Projections to the TSP Polytope

Michael Ben Or told me about the following great paper Linear vs. Semidefinite Extended Formulations: Exponential Separation and Strong Lower Bounds by Samuel Fiorini, Serge Massar, Sebastian Pokutta, Hans Raj Tiwary and Ronald de Wolf. The paper solves an old conjecture of Yannakakis about projections of polytopes.

From the abstract: “We solve a 20-year old problem posed by M. Yannakakis and prove that there exists no polynomial-size linear program (LP) whose associated polytope projects to the traveling salesman polytope, even if the LP is not required to be symmetric. Moreover, we prove that this holds also for the maximum cut problem and the stable set problem. These results follow from a new connection that we make between one-way quantum communication protocols and semidefinite programming reformulations of LPs.”

There are many interesting aspects to this story. The starting point was a series of papers in the 80s trying to prove that P=NP by solving TSP using linear programming. The idea was to present the TSP polytope as a projection of a larger dimensional polytope described by  polynomially many linear inequalities, and solve the LP problem on that larger polytope.  Yannakakis proved that such attempts are doomed to fail, when the larger LP problem keep the symmetry of the original TSP polytope.

Yannakakis asked if the symmetry condition can be removed and this is what the new paper shows. This is a very interesting result also from the point of view of convex polytope theory.

Another exciting aspect of the paper is the use of methods from quantum communication complexity.

Update: See this post over GLL for discussion and a description of a follow up paper.

# Test Your Intuition (12): Perturbing a Polytope

Let P be a d-dimensional convex polytope. Can we always perturb the vertices of P moving them to points with rational coordinates without changing the combinatorial structure of P?

In order words, you require that a set of vertices whose convex hull is a k-dimensional face of P will have this property after the perturbation.

# The Polynomial Hirsch Conjecture: Discussion Thread, Continued

Here is a  link for the just-posted paper Diameter of Polyhedra: The Limits of Abstraction by Freidrich Eisenbrand, Nicolai Hahnle,  Sasha Razborov, and Thomas Rothvoss.

And here is a link to the paper  by Sandeep Koranne and Anand Kulkarni “The d-step Conjecture is Almost true”  – most of the discussion so far was in this direction.

We had a long and interesting discussion regarding the Hirsch conjecture and I would like to continue the discussion here.

The way I regard the open collaborative efforts is as an open collective attempt to discuss and make progress on the problem (and to raise more problems), and also as a way to assist people who think or work (or will think or will work) on these problems on their own.

Most of the discussion in the previous thread was not about the various problems suggested there but rather was about trying to prove the Hirsch Conjecture precisely! In particular, the approach of Sandeep Koranne and Anand Kulkarni which attempts to prove the conjecture using “flips” (closely related to Pachner moves, or bistaller operations) was extensively discussed.  Here is the link to another paper by Koranne and Kulkarni “Combinatorial Polytope Enumeration“. There is certainly more to be understood regarding flips, Pachner moves, the diameter, and related notions. For example, I was curious about for which Pachner moves  “vertex decomposibility” (a strong form of shellability known to imply the Hirsch bound) is preserved. We also briefly discussed metric aspects of the Hirsch conjecture and random polytopes.

For general background: Here is a  chapter that I wrote about graphs, skeleta and paths of polytopes. Some papers on polytopes on Gunter Ziegler’s homepage  describe very interesting and possibly relevant current research in this area. Here is a  link to Eddie Kim and Francisco Santos’s survey article on the Hirsch Conjecture.

Here is a link from the open problem garden to the continuous analog of the Hirsch conjecture proposed by Antoine Deza, Tamas Terlaky, and  Yuriy Zinchenko.

# Igor Pak’s “Lectures on Discrete and Polyhedral Geometry”

It is a wonderful book, full of gems, contains original look on many important directions, things that cannot be found elsewhere, and great beyond great pictures (which really help to understand the mathematics). Grab it!

# Five Open Problems Regarding Convex Polytopes

## The problems

1. The $3^d$ conjecture

A centrally symmetric d-polytope has at least $3^d$ non empty faces.

2. The cube-simplex conjecture

For every k there is f(k) so that every d-polytope with $d \ge f(k)$ has a k-dimensional face which is either a simplex or combinatorially isomorphic to a k-dimsnional cube.

3. Barany’s question

For every d-dimensional polytope P and every k, $0 \le k \le d-1$,  is it true that $f_k(P) \ge \min (f_0(P),f_{d-1}(P))$?

(In words: the number of k-dimensional faces of P is at least the minimum between the number of vertices of P and the number of facets of P. )

4.  Fat 4-polytopes

For 4-polytopes P, is the quantity $(f_1(P)+f_2(P))/(f_0(P)+f_3(P))$ bounded from above by some absolute constant?

5.  five-simplicial five-simple polytopes

Are there 5-simplicial 5-simple 10-polytopes? Or at least 5-simplicial 5-simple d-polytope for some d?

(A polytope P is k-simplicial if all its faces of dimension at most k, are simplices. A polytope P is k-simple if its dual P* is k-simplicial.)

And on a personal note: My beloved, beautiful,  and troubled country celebrates 60 today: happy birthday!

Update (May 12): David Eppstein raised in a followup a sort of a dual question to Barany’s. For a d-polytope with n vertices and n facets what is the maximal number of k-faces. For a fixed d and large n the free join of pentagons is conjectured to give asymptotically the best upper bound.

Update (July 29) Gunter Ziegler reminded me of the following additional problem of Barany: Is the number of saturated chains in a d-polytope bounded by some constant (depending on d) times the total number of faces (of all dimensions) of the polytope. A saturated flag is a 0-face inside a 1-face inside a 2-face … inside a (d-1)-face.